Such a vaporiser typically comprises an elongate crucible arranged to contain a source of material, and a heater to heat the source to generate the feed gas. Typically, two such vaporisers are provided in a semiconductor implant apparatus, each containing a different source of material for generating different species of feed gas. In order to switch between feed gases, it is necessary to disconnect the power supply to the heater and allow the source to cool thereby ceasing the production of feed gas. The arc chamber is then purged before the heater of the second vaporiser is activated in order to commence the supply of the second feed gas. Usually, the vaporiser is allowed to cool by radiation, but in one prior art attempt to improve the cooling, a jet of N2 is directed at the end of the crucible on the atmospheric side. This improves the cooling to some extent, but does little to cool the source of material at the far end of the crucible.
A further problem is that although vaporisers used in the prior art operate well at or close to their maximum temperatures, it is sometimes necessary to operate vaporisers at a lower temperature. Control of conventional vaporisers at this lower temperature has proved to be difficult as the vaporiser is particularly sensitive to sudden power surges to the heater.